This patent application is related to copending application Ser. No. 850,234, filed Apr. 10, 1986 for DYNAMIC AIR GAP MEASURING DEVICE FOR USE WITH ROTATING ELECTRICAL MACHINERY filed concurrently herewith.
This invention relates generally to rotating electrical machinery, and more particularly the invention relates to the detection of loose coils in the stator of such machinery.
In the hydroelectric generator, voltage and current are induced in windings of the stator due to the rotation of the rotor and rotor windings within the stator. With the advent of epoxy insulation systems, the physical characteristics of generator stator windings changed. The former asphaltic insulation systems would swell over a period of time at temperatures reached during normal operation. This swelling locks the stator bar into the stator slot, thereby preventing relative movement caused by the high interacting magnetic forces. Epoxy insulation does not expand with time and temperature but instead tends to shrink slightly as the curing process continues. For this reason, it has been difficult to install a winding in a generator that will remain completely tight over a long period of time; this is particularly true for hydroelectric generators. As the stator winding components begin to loosen, the alternating magnetic forces cause the stator bars and stator wedges to "chatter" against the sides of the stator slot.
No technique has been available for determining when components in a hydroelectric generator stator become loose. Even though this is a major problem for the utility industry, the industry has had to rely on periodic inspections to locate the effects and damage caused by this phenomenon, thus a device and a method are needed to be able to detect this looseness before the insulation is worn off and the machine fails from an electrical short circuit.
The present invention is directed to detecting field winding looseness by acoustically monitoring noise or "chatter" created by the stator bars and stator wedges. On salient pole generators, there is generally sufficient room between pole pieces on the rotor to mount an acoustic monitor for the purpose of continuously monitoring the stator windings. If any components of the stator or winding are loose, the hammering of these components will tend to create acoustic energy at a frequency that is twice the rated frequency of the generator (e.g., 120 Hz for a 60 Hz generator). The acoustic energy is picked up by microphones that would be developed especially for this application and environment. The acoustic transducer must be immune to external electromagnetic radiation.
In order to cover a wide range of acoustical frequencies, the monitor scans a range from DC up to about 10 KHz. The source of the sound created by loose components is determined by a specific timing reference associated with the rotation of the rotor and the known doppler shift in frequency as the rotor-mounted sensor approaches the sound source and as the sensor departs the sound source.
Briefly, an acoustic monitor in accordance with the invention comprises an acoustic transducer mounted on the rotor and oriented to receive acoustic signals from the stator windings. The transducer converts the acoustic signals to electrical signals which can be processed to identify loose windings.
In a preferred embodiment the electrical signals from one or more rotor mounted monitors are amplified and then applied to a voltage to frequency converter for conversion to pulses for transmission. The pulses are applied to a suitable output driver such as an FM modulator, and the output of the driver is then applied to an RF transmitter on the rotor for coupling to a receiver. The receiver transmits the signal to computer means for analysis. Control signals from the computer means are coupled to the rotor for sensor control through a multiplexer in the case of a plurality of monitors.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims.